This isn't defined for Gaussian ECPs. The core energy you are referring to with VASP is the core reconstruction within the PAW formalism. Gaussian ECPs are constructed differently. The advantage you have with Gaussian basis sets is that you can explicitly do an all-electron calculations albeit with a higher cost. You also have to be specific on what you define as a core. Gaussian ECPs typically include more electrons in the valence space (outer valence and semi-core states). This is not typically done in plane wave based pseudopotentials.